Fucoidans are high-value sulfated polysaccharides derived from brown seaweed. Fucoidans were reported to possess various biological effects such as anti-inflammatory, anticoagulant, antioxidant and antitumor. However, the presence of alginate (as a co-extractant) in fucoidan polysaccharide extracts causes the low purity of fucoidan. Alginate is water-soluble in the form of sodium alginate, while it is insoluble in the form of calcium alginate, therefore, the addition of calcium will promote alginate precipitation during the purification process. A green and sustainable technique was invented to purify fucoidan and alginate from Sargassum sp. using corals, which is rich in calcium carbonate. The corals adsorb alginates during extraction of fucoidan, thus producing higher purity fucoidan. The adsorbed alginates were then desorbed using sodium chloride solution, producing higher purity alginate. The corals treated with sodium chloride are reusable for fucoidan and alginate extraction